Due to the success of the Internet, the Internet Protocol (IP) has become the primary networking protocol. Major concerns of the Internet community are the depletion of global IP address space (IPV4) and the complexity of configuring hosts with global IP addresses for Internet access. To extend the life of current IP address space and provide configureless access, network address translation (NAT) and its extension, port address translation (PAT), have been employed.
Network address translation supports connectivity between the Internet and hosts using private addressing schemes. This connectivity provides configureless access to the Internet in that hosts may have independently assigned, globally non-unique addresses that need not be coordinated with the Internet Address Numbering Association (IANA) or other Internet registry. Network address translation pairs up the private addresses to public addresses so that the inside IP addresses appear as legally registered IP addresses on the Internet.
Port address translation allows a number of private network addresses and their ports to be translated to a single network address and its ports. Thus, multiple hosts in a private network may simultaneously access the Internet using a single legally registered IP address. The registered IP address is typically assigned to a router that translates addressing information contained in message headers between the addressing schemes.
Port address translation uses transport layer header information (protocol, port, etc.) to uniquely translate and direct IP traffic to the correct receiver. IP fragments, other than the first fragment, however, do not carry any transport layer protocol information. Thus, when IP fragments are delivered out-of-order, which is common with IP traffic, the IP fragments cannot be translated unless the first fragment of the packet is already received. As a result, the fragments are discarded and the data must be resent. This leads to delays in obtaining information and increased traffic on the Internet.